A nuclear weapon is difficult to detect, particularly if it is shielded and hidden among cargo. Depending on the shield design, some radiation, notably gamma rays and neutrons, can leak out. In addition, the amount of time available for an inspection of vehicles and cargo, at an entry port for example, is generally limited to a short period such as one minute so as to not delay the flow of commerce. Unless the threat is emitting a large amount of radiation, the scan may fail to detect the threat in the available inspection time.
If a weapon successfully evades an initial radiation inspection, an adversary can then emplace it in an urban environment or a government center or near a military base, and use it for unlimited extortion or a future attack. Detecting and localizing such a hidden weapon would require a system capable of separating source particles from backgrounds as well as localizing the weapon spatially. Large-area detectors are necessary to intercept a sufficient fraction of the emitted radiation, and advanced designs are needed for directionality.
What is needed, then, is an advanced large-area detector system, capable of detecting gamma rays or neutrons or both, with sufficient detection efficiency to reveal even a very well-shielded nuclear weapon. Preferably the new system should also indicate the position of the weapon, despite clutter and backgrounds and active obfuscation, in a practical scan time, and preferably at minimal cost.